Advanced light extraction structure

ABSTRACT

This presently disclosed technology relates Organic Light Emitting Diodes (OLEDs), more particularly it relates to OLED display light extraction and nanocomposite formulations that can be used for the light extraction structure.

This application is a continuation of U.S. application Ser. No.14/903,822, filed Jan. 8, 2016 (now U.S. Pat. No. ______) which is theU.S. national phase of International Application No. PCT/US2014/045671filed Jul. 8, 2014 which claims benefit of and is a continuation-in-partof U.S. application Ser. No. 14/120,419, filed Jul. 8, 2013 (now U.S.Pat. No. 10,033,014) the entire contents of each such prior filedapplication being expressly incorporated hereinto by reference.

This presently disclosed technology relates Organic Light EmittingDiodes (OLEDs), more particularly it relates to OLED display lightextraction and nanocomposite formulations that can be used for the lightextraction structure.

Light Emitting Devices

Light Emitting Diodes (LEDs) and Organic Light Emitting Diodes (OLEDs)have enjoyed a rapid development in the past couple of decades and havestarted to replace existing lighting and display devices.

OLED devices are frequently grouped into “bottom emitting” OLEDS, whichemit light through a transparent substrate on which the OLED is built,and “top emitting” OLEDs, which emit light away from the substrate onwhich the OLED is built. Some OLEDs are patterned to form an array ofindividually addressable pixels (picture elements) or sub-pixels (one ofseveral neighboring emitters of different colors that are groupedtogether as a pixel but are individually addressable). Such pixelatedOLEDs are increasingly popular for use in digital display devices. Incontrast to pixelated OLEDs, other OLEDs are designed to have only oneemitting area, which may be small and narrow or large and extendeddepending on the intended application.

Due to the specific device structures of LEDs and OLEDs, significantportion of the light generated inside the active region is totallyreflected at various interfaces and is “trapped” inside the device,leads to reduced external efficiency of the light emitting device.

The external efficiency is defined as the power of all optical radiationemitted by the device divided by the total electrical power consumed bythe device. External efficiency is an important factor and affects suchdevice characteristics as power consumption, luminance, and lifetime.

The problem is particularly severe for OLED given the technology is in amuch earlier development stage than its LED counterpart. For example,only ˜20% of all the photons generated in an OLED lighting device areextracted out. Many light extraction schemes have been applied to LEDsand OLEDs, such as backside reflector, high refractive indexencapsulant, surface roughening or surface texturing, etc. Texturedextraction film is a popular solution for OLED lighting as it iscompatible with the roll-to-roll manufacturing process and can be easilyapplied on either side of the final encapsulation layer.

A pedagogical depiction of the device structure of a typical OLED devicewith textured surface is shown in FIG. 1. The active area (101) emitslight, for both a top emitting and bottom emitting device structure,through a transparent conductor, such as an Indium Tin Oxide (ITO),layer (102) and the substrate (103), which is surface textured (104) toreduce the light loss due to total internal reflection at thesubstrate/air interface.

In an Active Matrix OLED (AMOLED) display or Passive Matrix OLED(PMOLED), however, due to pixelated nature of the active region, thesurface texture degrades the optical quality of the pixels, creating anundesirable blur effect.

In one aspect of the presently disclosed technology, a light extractionstructure is described that can be placed immediately on top of, or inclose vicinity or proximity, of or to the active region. Such astructure can improve the light extraction of the OLED display and atthe same time preserve the optical quality of the pixels.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an exemplary OLED device structure with textured surface.

FIG. 2 shows an exemplary OLED device structure of current invention.

FIG. 3 shows an exemplary OLED pixel of presently disclosed technologyusing a hyper-hemispherical lens.

FIG. 4a : UV absorption spectrum of film from formulation(ZrO₂-(2-[2-(2-9-methoxyethoxy) ethoxy]acetic acid) in 70:30 BMA-TMPTA)after post bake at (401) 120° C. for 3 minute in air, (402) thermal bakeat 175° C. for 1 hour under N₂.

FIG. 4b : UV transmission spectrum of film from formulation(ZrO₂-(2-[2-(2-9-methoxyethoxy) ethoxy]acetic acid) in 70:30 BMA-TMPTA)after post bake at (403) 120° C. for 3 minute in air, (404) thermal bakeat 175° C. for 1 hour under N₂.

FIG. 5a : UV absorption spectrum of film from formulation(ZrO₂-(2-[2-(2-9-methoxyethoxy) ethoxy]acetic acid) in 70:30 BMA-TMPTA)after post bake at (501) 120° C. for 3 minute in air, (503) thermal bakeat 200° C. for 1 hour under N₂.

FIG. 5b : UV transmission spectrum of film from formulation(ZrO₂-(2-[2-(2-9-methoxyethoxy) ethoxy]acetic acid) in 70:30 BMA-TMPTA)after post bake at (504) 120° C. for 3 minute in air, (505) thermal bakeat 200° C. for 1 hour under N₂.

FIG. 6a : UV absorption spectrum of film from formulation(ZrO₂-(2-[2-(2-9-methoxyethoxy) ethoxy]acetic acid) in 70:30 BMA-TMPTA)after post bake at (601) 120° C. for 3 minute in air, (604) thermal bakeat 200° C. for 2 hour under N₂.

FIG. 6b : UV transmission spectrum of film from formulation(ZrO₂-(2-[2-(2-9-methoxyethoxy) ethoxy]acetic acid) in 70:30 BMA-TMPTA)after post bake at (605) 120° C. for 3 minute in air, (606) thermal bakeat 200° C. for 2 hour under N₂.

DETAILED DESCRIPTION

A first exemplary embodiment of a light extraction structure may containan array of lenses or set of lenses, the array of lenses or sets oflenses contain a high refractive index material having refractive indexhigher than the encapsulation layer or the substrate, each of the lensor set of lenses is applied between the active region of a lightemitting device and the encapsulation layer or substrate layer of thelight emitting device, each of the lens or set of lenses covers at leastone pixel, a planarization layer between the array of lenses or sets oflenses and the encapsulation or substrate layer, the light extractionstructure enhances the overall extraction efficiency of light generatedby the active region to the viewer or the external light detector.

A light extraction structure of the present disclosure may include anarray of lenses and optionally or additionally a planarization materialas described herein.

An illustrative depiction of an embodiment of the presently disclosedtechnology is shown in FIG. 2.

The active layer (201) is divided into an array of pixels (202). Thelight emitted from the pixels may pass through a thin ITO layer (203)and the lenses (204). A planarization layer (205) may be applied toreduce air trapped between the lenses and the substrate, promote betteradhesion, and provide refractive index matching. The light transmits outof the substrate (206).

The light emitting device of an exemplary embodiment of a lightextraction structure of the presently disclosed technology may containlight emitting diode(s) (LED), organic-light emitting diode(s) (OLED),electro-luminescence device(s), or liquid crystals device(s) (LCD).

Light extraction efficiency may be improved in the presently disclosedtechnology by incorporation or inclusion of a lens material that iscompatible with current process and material system, has a higherrefractive index than the substrate or encapsulant, and/or a highoptical transparency in the visible spectrum.

Lens and light extraction structures of the presently disclosedtechnology achieve these requirements by incorporating and/or includingnanocrystals and/or nanocomposites as described herein.

According to the presently disclosed technology, when the size ofnanocrystals (such as inorganic nanocrystals) described and/or includedin the lens and/or light extraction structure(s) of the presentlydisclosed technology is smaller than one tenth of the wavelength of thelight, the scattering by the nanocrystals is negligible. Therefore, highrefractive index, high transparency nanocomposites of the presentdisclosure can be achieved by dispersing inorganic nanocrystals withhigh refractive index into polymeric materials with relatively lowerrefractive index, while at the same time meeting the processabilityrequirements of many manufacturing process involving lighting structureswhere the presently disclosed lens and/or light extraction structuresmay be incorporated.

One example of such a high refractive index nanocomposite is disclosedin U.S. Provisional Patent Application No. 61/790,156, filed Mar. 15,2013 and U.S. patent application Ser. No. 12/211,971, filed Mar. 14,2014, the entire contents of each of which are incorporated herein intheir entirety by reference. In this material system, mono-dispersedsub-10 nm ZrO₂ nanocrystals with surface capping agents are dispersed inacrylic monomers, that can be further cured with UV-light to form a highrefractive index coating.

The high refractive index material of a light extraction structure ofthe presently disclosed technology may contain a nanocomposite orformulation, that contains inorganic nanocrystals and a polymericmatrix.

The nanocomposite or formulation may contain curing agent(s) and/orphoto-initiator(s), and may be UV curable. Additionally oralternatively, the nanocomposite of formulation may contain curingagent(s) and may be thermally curable.

The nanocomposite or formulation may additionally contain a plasticizer,and/or toughener, and/or thickener, and/or thinner, and/or surfactant,and/or flexibilizer, and/or anti-color agent, and/or other functionaladditive(s).

Nanocrystals of nanocomposite(s) or formulation(s) of the presentlydisclosed technology may contain ZrO₂, TiO₂, ZnO, MgO, HfO₂, Nb₂O₅,Ta₂O₅, and/or Y₂O₃. These inorganic materials possess both highrefractive index and transparency at visible spectrum.

The nanocrystals of nanocomposite(s) or formulation(s) of the presentlydisclosed technology may have size smaller than 10 nm in at least onedimension.

The nanocrystals of nanocomposite(s) or formulation(s) of the presentlydisclosed technology optionally include specific functional group(s),such as capping agent(s) or capping group(s). These specific functionalgroup(s) have been grafted to the surface of the nanocrystals of thepresently disclosed technology. Such nanocrystals are described hereinas well as in U.S. Pat. No. 8,592,511 (Williams et al), the entirecontent of which is incorporated in its entirety herein by reference.

Exemplified capping agent(s) demonstrated in the present disclosureinclude 2-[2-(2-9-methoxyethoxy) ethoxy]acetic acid and/ormethoxy(triethyleneoxy) propyltrimethoxysilane and/or3-methacryloyloxypropyltrimethoxysilane and/or n-octyl trimethoxysilaneand/or dodecyltrimethoxysilane and/or m,p-ethylphenethyltrimethoxysilane.

Capping of nanocrystals may require a solvent exchange as as-synthesizednanocrystals may be surface modified in a solvent other than the solventof nanocrystals synthesis. Solvent exchange may be accomplished by, forexample, decanting reaction liquor and rinsing the nanocrystals with thecapping solvent, which may then be used as a washing or rinsing solventthat is itself decanted to produce a wet cake of uncapped nanocrystals.

For example to perform the surface modification of the nanocrystals with2-[2-(2-9-methoxyethoxy) ethoxy]acetic acid, the nanocrystals may besuspended in the capping solvent, for example, toluene for2-[2-(2-9-methoxyethoxy) ethoxy]acetic acid modification, alternativelyat a loading of 10 wt % or greater, alternatively 20 wt % or greater,alternatively 30 wt % or greater, calculated based on the weight of thewet nanocrystal cake. While the suspension is being stirred, the cappingagent may be added slowly. The amount of capping agent used may be 8-60wt % to the weight of the wet nanocrystal cake. The suspension may beallowed to stir at 20-27° C. for 10-30 minutes and then refluxed at theboiling point of the capping solvent for 30-60 minutes. After refluxing,the clear solution is cooled to 50-60° C. slowly. This suspension maythen be filtered to remove dust and aggregates larger than 200 nm. Thecapped nanocrystals may then be precipitated out from the cappingsolvent using heptane. The precipitated nanocrystals may then becollected by centrifugation. The nanocrystals thus collected may then bedispersed in tetrahydrofuran (THF) and again re-precipitated usingheptane. This process may be repeated twice. The wet cake ofnanocrystals collected in the final step may then be dried under vacuum.

The presently disclosed nanocomposite(s) or formulation(s) may also bemade as a solvent free formulation or as a formulation or material witha low or reduced solvent content. Such low or no solvent materials aredesirable both because of environmental and health purposes and becauseof processing constraints and/or limitations when solvents are present.

Inorganic nanocrystals of the present disclosure are, for example,mono-dispersible, with an average size range of 3-7 nm, and containing asurface treatment that aids in dispersion of the nanocrystals in a largevariety of solvents and polymers. The presently disclosed materialadvantageously does not require the inclusion of solvents and thenanocrystals of the present disclosure are dispersible in the polymerand/or monomer material of the present disclosure, without the inclusionof solvents or additional dispersing agents. These nanocrystals, whichhave been surface modified with capping agents, possess greatercompatibility with monomers and/or oligomers and/or polymers withoutreduction in processability. The surface modified nanocrystals of thepresent application may be formulated in a resin mixture that istransparent and has a viscosity that allows spin coating of, forexample, 3-4 micron thick films. The films obtained from thenanocomposite(s) or formulation(s) of the present disclosureadditionally demonstrate high refractive index, high opticaltransmittance in the visible spectrum, and are thermally stable attemperatures above 120° C., or above 175° C., or above 200° C.

The films or light extraction structure(s) including thenanocomposite(s) or formulation(s) according to the present disclosurepossess a high refractive index of 1.6 and higher at 400 nm, or 1.7 andhigher at 400 nm, or 1.8 and higher at 400 nm, or 1.9 at 400 nm. Therefractive index of the films according to the present disclosure mayrange from 1.6 to 1.9 at 400 nm.

The films or light extraction structure(s) including thenanocomposite(s) or formulation(s) of the present disclosureadditionally or alternatively possess high optical (440-800 nm)transmittance of 80% or 82%, or 86%, or 88%, or 90%, or 92%, or 94%, or96%, or 98%, and above for films that are less than 5 microns thick. Thefilms of the present disclosure therefore possess a high optical(440-800 nm) transmittance in the range of 80% to 98% and above forfilms that are less than 5 microns thick.

The transmittance of a film according to the present disclosure may bemeasured with a Perkin-Elmer UV-Vis Lambda spectrophotometer, whereinthe film is coated on a fused silica substrate and another blank fusedsilica of the same quality and thickness is used as a reference. FIG. 4a, FIG. 4b , FIG. 5a , FIG. 5b , FIG. 6a , and FIG. 6b are examples ofthe absorbance and transmission results of these films. The ripplesshown in these curves are the results of interference of the incominglight and the reflected light at the film/substrate interface.

An exemplary non-limiting embodiment of a formulation of the presentdisclosure comprises or contains a mixture of acrylic monomers and/oroligomers, and capped or surface treated zirconium oxide nanocrystals.The loading or amount of the nanocrystals included in a formulation ofthe present disclosure is in the range of 50 wt % to 90 wt % based onthe weight of the entire formulation, such as a loading of 50 wt % orgreater, or 55 wt % or greater, or 60 wt % or greater, or 65 wt % orgreater, or 70 wt % or greater, or 75 wt % or greater, or 80 wt % orgreater, or 90 wt %.

The polymer matrix may comprise or contain acrylic monomers, such asbenzyl methacrylate (BMA) and trimethylolpropane triacrylate (TMPTA),that optionally included or combined or mixed in a mass ratio in therange of 75:25 to 65:35 wherein the BMA may be present in a relativerange of 65-75 and the TMPTA may be present in a relative range of25-35.

The physical properties of TMPTA, such as viscosity, low volatility andrefractive index, make the material uniquely advantageous in a materialor composition or film or coating of the present disclosure. TMPTA isless viscous, for example, than hexamethylene diacrylate (HMDA) andbisphenol A diglycerolate dimethacrylate but more viscous thandivinylbenzene (DVB). Of the two, TMPTA and HMDA, TMPTA has the higherrefractive index (RI=1.474 and 1.456 for TMPTA and HMDA respectively).

BMA is unique in the composition, material and film of the presentdisclosure in that the monomer has a high refractive index (for anmonomer or polymer) of 1.512. The refractive index of BMA thereforehelps increase the final refractive index of the film.

Dispersing nanocrystals in BMA alone or with the aid of a solvent suchas propylene glycol methyl ether acetate (PGMEA) resulted in films thatare difficult to cure by UV or were cracked upon heating at 120° C. andabove.

Another multifunctional acrylic monomer, such as TMPTA, HMDA, DVB orbisphenol A diglycerate dimethacrylate (Bisphenol A) may be added as apotential additive to increase the viscosity of the formulation. Filmsfrom HMDA-BMA, DVB-BMA and Bisphenol A-BMA combinations were foundhowever to be too brittle in formulations containing nanocrystals of thepresent disclosure such that these films of these combinations crackedwhen heated at 120° C. or above.

Additionally, TMPTA and HMDA have refractive indexes<1.49; such thatincluding these monomers reduces the refractive index of the finalformulation and film product when compared with BMA.

As described herein, the specific combinations of BMA, TMPTA andnanocrystals of the present disclosure, in the ratios and amountsdescribed herein, provide unexpected advantages in a combination ofphysical properties, including but not limited to refractive index,light transmittance, temperature resistance and viscosity.

A mass ratio of BMA to TMPTA in the range of 75:25 to 65:35 as describedherein has also been discovered to provide unique and unexpectedadvantages, i.e. high refractive index, high transmittance, and hightemperature resistance, in the formulations or compositions of films ofthe present disclosure. While materials and/or films containing massratios of BMA to TMPTA ranging from 95:5 to 80:20 (i.e., 95:5, 90:10 and80:20) with nanocrystal loading of 80 wt % and above were stable attemperatures below 120 C, as shown in Table 1 below. Nanocrystals of thepresent disclosure dispersed in TMPTA, without BMA, provided a lowerrefractive index material than with BMA. Films produced from a massratio of BMA to TMPTA according to the presently disclosed technologydemonstrated enhanced film quality with, for example, reduced surfaceroughness and thicker films due, at least in part, to higher viscosity.

TABLE 1 Film results of capped ZrO₂ nanocrystals in monomer mixture.′Good′ indicates that the film does not yellow or crack when heated atthose indicated temperatures. ′Cracked′ indicates that the film crackedduring thermal baking. Disadvantage of this formulation is that itcomprises of PGMEA to aid in the solubility. Content of Post Post Postbaked ZrO₂ to baked at baked at at 200 C./ Monomer mix monomer Cappingagent 120 C./60/air 175 C./60/N₂ N2/60 min 2 - 10 wt % Bisphenol 50 - 80wt % 2-[2-(2-9- good cracked A diglycerolate methoxyethoxy)dimethacrylate in ethoxy]acetic acid BMA 2 - 25 wt % TMPTA in 50 - 80 wt% methoxy(triethyleneoxy) good cracked BMA propyltrimethoxysilane and 3-methacryloyloxypropyl trimethoxysilane 25 - 30 wt % TMPTA 50 - 80 wt %methoxy(triethyleneoxy) good good cracked in BMA propyltrimethoxysilaneand 3- methacryloyloxypropyl trimethoxysilane 20 - 30 wt % TMPTA 50 - 80wt % 2-[2-(2-9- good good good in BMA methoxyethoxy) ethoxy]acetic acid25 - 30 wt % TMPTA 50 - 80 wt % methoxy(triethyleneoxy) good crackedcracked in BMA propyltrimethoxysilane 25 - 30 wt % TMPTA 82 - 86 wt %2-[2-(2-9- good cracked cracked in BMA methoxyethoxy) ethoxy]acetic acid

The photo-initiator may comprise or contain benzophenone, optionally inan amount of 1-5 wt % based on the total weight of the formulation orcomposition or material of the present disclosure. Such aphoto-initiator may be mixed or included or dissolved or dispersed inthe monomer and/or oligomer and/or polymer mix of the presentlydisclosed formulation by means known in the art, such as by stirring orvortexing at temperature of, for example, in the range of 20-30 C.

While benzophenone has been exemplified herein as a photo initiator,other photo initiators can also or otherwise be employed depending on,for example, curing time and lamp type. Other photo initiators of thepresent disclosure include Speedcure BEM and TPO(diphenyl(2,4,6-trimethylbenzoyl)-phosphine oxide), which may allow forconsiderable reduction in the required UV exposure time.

A nanocomposite or formulation of the present disclosure optionally hasa viscosity of less than 12,000 Cps at 20° C. as measured by aBrookfield RVDV-II+PCP cone and plate viscometer. A nanocomposite orformulation of the present disclosure additionally or alternatively hasa transmittance higher than 60% at a wavelength of 400 nm as measured bya Perkin Elmer Lambda 850 Spectrophotometer in a 1 cm path lengthcuvette. A nanocomposite and composition of the present disclosurecontains or comprises an organic mixture of benzyl methacrylate andtrimethylolpropane triacrylate. Such a nanocomposite, composition of thepresent disclosure optionally contains or comprises a weight ratio ofbenzyl methacrylate to TMPTA in the range of 75:25 to 65:35.

A nanocomposite or formulation or film of the present disclosureoptionally and/or additionally possesses a refractive index of greaterthan 1.8 at 400 nm.

In some embodiments the nanocomposite or formulation of the presentdisclosure does not include the purposeful addition of solvents.

The polymer matrix may comprise acrylic, epoxy, silicone, siloxane,polycarbonate, polyurethane, polyimides,Poly(3,4-ethylenedioxythiophene) (PEDOT), poly(styrene sulfonate) (PSS)doped PEDOT, Polyethylene terephthalate (PET), Polyethylene naphthalate(PEN), or doped poly(4,4-dioctylcyclopentadithiophene), and theircorresponding monomers and/or oligomers.

Another example of a method of forming a nanocomposite of the presentdisclosure includes mixing an epoxy resin, such as resin EPON 862, andcuring agent W (or curing agent 3295, or the like), such as by handusing a weight ratio of 5:1. To this mixture ZnO or ZrO2 capped withmethoxytri(ethyleneoxy)propyltrimethoxysilane is then added. The weightratio of the nanocrystals to the epoxy mixture can be in the range offrom 1:1000 to 10:1. A small amount of THF (no more than 200 wt % of thecomposite mixture) can be added to reduce the viscosity of thenanocrystal/epoxy resin mixture. The mixture is then sonicated eitherinside a sonication bath or using a Hielscher UP200S sonication probefor less than five minutes. After sonication, the composite mixture (2gram to 4 grams) was then poured into an aluminum pan (4 cm diameter),which acted as a mold. The loaded pan was and placed inside a vacuumoven. Vacuum was applied in order to remove the THF and air bubbles. Theoven was then heated to 80° C. for overnight (>10 hr) under vacuum. Theresulting composite was post cured at 150° C. for another 3 hours beforeit was removed from the vacuum oven.

Another example of a method of forming a nanocomposite of the presentdisclosure may be as follows: epoxy resin EPON 862 and curing agent 3274were pre-mixed by hand using weight ratio of 10:4. 3-(methacryloyloxy)propyl trimethoxysilane capped ZrO2 nanocrystals are then added into theepoxy resin at loading levels between 0.01-99.99 wt %. A small amount ofacetone (no more than 200 wt % of the composite mixture) was added toreduce the viscosity of the nanocrystal/epoxy resin mixture. The mixtureis then sonicated either inside a sonication bath or using a HielscherUP200S sonication probe for less than five minutes. The mixed compositemixture (2 gram to 4 grams) was then poured into an aluminum pan (4 cmdiameter), which acted as a mold. The loaded pan was then placed insidea vacuum oven. Vacuum was applied to remove the acetone and air bubbles.The resulting composite was cured at room temperature for 24 hoursbefore it was removed from the vacuum oven.

For spin coating 3-(methacryloyloxy)propyl trimethoxysilane cappednanoparticle/epoxy composite films, a typical protocol is described asfollows: epoxy resin EPON 862 and curing agent 3274 were premixed byhand using weight ratio of 10:4. The desired amount of cappednanocryastals is then added into the epoxy resin at loading levelsbetween 1-99.99 wt %. Acetone was added to prepare a spin solution withan appropriate solid content (ranging from 10 wt % to 50 wt %). Themixture is then sonicated inside a sonication bath for 5 minutes. Thesolution can then be used directly for spin-coating. By varying thespin-rate different film thicknesses ranging from several hundrednanometers to several micrometers may be achieved.

Each lens or set of lenses of the exemplary light extraction structuremay cover a single pixel of the light emitting device.

Each lens or set of lenses of the exemplary light extraction structuremay cover multiple pixels of the light emitting device.

Each lens or set of lenses of the exemplary light extraction structuremay comprise a single lens element, said lens element may comprisespherical, semi-spherical, hyper-semispherical, parabolic, concave,convex, sub-wavelength pyramid array, surface texture, or any othersurface curvature, or Fresnel lens.

Each lens or set of lenses of the exemplary light extraction structuremay comprise a single lens element, said lens element comprises a gradedor gradient index profile along at least one dimension of the lens, saidgraded or gradient lens may comprise curved surface.

Each lens or set of lenses of the exemplary light extraction structuremay comprise multiple lens elements, said lens elements may comprisesinglet lens, lens with graded or gradient index profile, achromaticlens doublet, prism, filter, polarizer, reflector, Fresnel lens, or anyother common optical elements.

Each lens or set of lenses of the exemplary light extraction structuremay be separated from the active region of said light emitting device byless than the wavelength of the highest energy photons emitted by saidlight emitting device.

Another exemplary method of making a of a light extraction structure fora light emitting device comprises: an active region, an array of lensesor set of lenses, said array of lenses or sets of lenses comprise a highrefractive index material having refractive index higher than thesubstrate or the encapsulation layer, said lenses or sets of lenses areapplied between the active region of a light emitting device and theencapsulation layer of said light emitting device, a planarization layerbetween said array of lenses or sets of lenses and said encapsulation orsubstrate layer, said light extraction structure enhances the overallextraction efficiency of light generated by the active region to theviewer or the external light detector.

The applying in the exemplary method of making a light extractionstructure comprises applying a prefabricated sheet comprising the saidarray of lenses or sets of lenses on top of the active region of saidlight emitting devices.

The applying a prefabricated sheet may comprise roll-to-roll printing.

The applying in the exemplary method of making a light extractionstructure comprises applying a layer of the high refractive indexmaterials on top the active regions, by spin-coating, dip-coating, bladecoating, draw-bar coating, slot-die coting, spraying, or any othercommon coating techniques, and then forming said array of lenses or setof lenses through imprint lithography, optical lithography, or anotherother common patterning techniques.

The apply in the exemplary method of making a light extraction structurecomprises UV curing.

The apply in the exemplary method of making a light extraction structurecomprises thermal curing.

Examples

One example light extraction structure comprises an array of hyperhemispherical, lens centered on an active region. For illustrationpurpose, one unit of such a structure is shown in FIG. 3. The structureis similar to FIG. 2, the active region (301) is divided into pixels(302). For simplicity, only one pixel is shown here. The pixel or pixelshaving a refractive index of n3. An ITO layer (303) may exist betweenthe pixel and the lens. The lens is shaped into a hyper-semi-sphere(304) with h=R/n2, where R is the radius of the semi-sphere and n2 isthe refractive index of the lens, also known as Weierstrauss geometry. Afiller, or a planarization layer (305), with refractive index n1, may beapplied between the lens and the substrate (306).

In the case n3>n2>n1, it can be shown with simple ray tracing, that forboth hyper semispherical and spherical, which is a special case withh=0, lenses can significantly improve the light extraction from thepixels. And the geometry with h=R/n2 offers the highest collectionefficiency. The hyper semispherical lens offers an extra benefit in thatfor an emitter located at the center of the lens, it focuses the emittedlight to a smaller solid angle, as shown in FIG. 3. 309 represents therays without any lens or with a spherical lens, while 308 represents therays after the hyper semispherical lens. For an optical system with alimited numerical aperture, in this case the numerical aperture islimited by the total internal reflection at the substrate/air, thisability makes hyper semispherical lens efficient in coupling into thesubstrate/air interface escape cone.

Another example light extraction structure comprises an array ofhemispherical lens centered on an active region, in a similar structureas in FIG. 2, with 204 being hemispherical lenses. Such a systemprovides higher light coupling compared with the system in FIG. 1without surface texturing (104).

Another example light extraction structure comprises an array ofhypo-spherical lens centered on an active region, in a similar structureas in FIG. 2, with 204 being hypo-spherical lenses. In hypo-sphericallens, h is negative. Such a system still provides higher light couplingcompared with the system in FIG. 1 without surface texturing (104).

In one example of an exemplary non-limiting formulation, acrylicmonomers, benzyl methacrylate (BMA) and trimethylolpropane triacrylate(TMPTA), was mixed in a mass ratio of 70-75 to 25-30. 1-5 wt % ofbenzophenone as photo initiator, was dissolved in the monomer mix eitherby stirring or vortexing at temperature of 20-30 C. The solution wasthen filtered to remove dusts and then added to dry ZrO₂ nanocrystal andallowed to soak in the monomer blend until no ZrO₂ powder was observed.In large scale, gently shaking the dried nanocrystals with the monomerblend is acceptable. Once all ZrO₂ nanocrystals powder was completelydispersed in BMA-TMPTA, the viscous suspension was mixed for 10-15hours. Finally, the viscous suspension was filtered before processingthe film.

The suspension was validated by coating films and characterizing thephysical properties of the films such as thermal stability andtransmittance.

As a standard method, the suspension was coated on a 2″ silicon wafer orfused silica wafer to inspect its quality. The wafers were cleanedbefore applying the film to remove contaminants and dusts. 3-4 micronthick film was spin coated on silicon wafer at 1000-4000 rpm for 1-5minute.

An optional pre-bake process at 90° C. may be performed to remove theresidual solvent if that is a concern. In these formulations the solventis typically less than 10 wt %, more preferably less than 1 wt %. Thefilm was inspected for defects from undispersed particles or airbubbles. If no defects were observed, its surface roughness is measuredusing a surface profilometer.

The film coated on glass slide or fused silica wafer was cured by UVexposure for 60-200 seconds using a Dymax EC-5000 system with a mercury‘H’ bulb and then post-baked for 2-5 minutes at 120-150° C. under air.Further, the thermal stability of the film was tested by heating thefilm at a temperature of 175° C. or above, optionally about 200° C.,under nitrogen atmosphere for 1-2 hours. A crack free, colorless film isdesirable and indicates a good formulation.

These film demonstrate a refractive index of 1.80 or greater at 400 nmand transmittance>89% at 400 nm.

The refractive index is measured with a Woollam M-2000 spectroscopicellipsometer in the spectral range from 350 nm to 1700 nm and thetransmittance was measured using a Perkin Elmer Lambda 850Spectrophotometer.

This example formulation with 65-75:25-35 mass ratio of BMA to TMPTAwith nanocrystal loading of 50 wt % and above produced films that are UVcurable and can withstand a thermal baking at 200 C for 1-2 hour undernitrogen, as shown in Table 1.

Films spin coated from formulation containing zirconium oxidenanocrystals capped with 2-[2-(2-9-methoxyethoxy) ethoxy]acetic acid at50-80 wt % loading in the BMA-TMPTA (65-75:25-35 mass ratio) were stableand did not crack when heated at temperatures up to 200° C. However,films from formulation containing zirconium oxide nanocrystals cappedwith 2-[2-(2-9-methoxyethoxy) ethoxy]acetic acid at 82-85 wt % loadingin the BMA-TMPTA (65-75:25-35 mass ratio) were stable only attemperatures below 120° C., as shown in Table 1. Also, zirconium oxidenanocrystals modified with other capping agents such asmethoxy(triethyleneoxy) propyltrimethoxysilane and/or3-methacryloyloxypropyltrimethoxysilane and/or n-octyl trimethoxysilaneand/or dodecyltrimethoxysilane and/or m,p-ethylphenethyltrimethoxysilane formed good dispersions in BMA-TMPTA mixture, as wellas good films, but was only stable up to 120° C.

One advantage of this exemplary non-limiting embodiment is that bothmonomers are in liquid form at room temperature so no solvent isnecessary at room temperature and the film is UV curable. Surfacemodified ZrO₂ nanocrystals are dispersed directly in the monomer. Such adirect dispersion eliminates, for example, the need to remove thesolvent at a later step.

Nanocrystals of the exemplified embodiments of the present disclosurehave been surface modified with various capping agents such as2-[2-(2-9-methoxyethoxy) ethoxy]acetic acid and/ormethoxy(triethyleneoxy) propyltrimethoxysilane and/or3-methacryloyloxypropyltrimethoxysilane and or n-octyl trimethoxysilaneand/or dodecyltrimethoxysilane and/or m,p-ethylphenethyltrimethoxysilane. In an exemplified method of producing the cappednanocrystals of the present disclosure, the as-synthesized nanocrystalsare allowed to settle for at least 12 hours after synthesis. Since thenanocrystals are surface modified in a solvent other than the synthesissolvent, the nanocrystals are separated from the reaction liquid bydecanting off the reaction liquid and rinsing the nanocrystals with thecapping solvent. The rinsing solvent is decanted off to obtain a wetcake of uncapped nanocrystals.

For the surface modification of the nanocrystals with2-[2-(2-9-methoxyethoxy) ethoxy]acetic acid, the nanocrystals aresuspended in the capping solvent, for example, toluene for2-[2-(2-9-methoxyethoxy) ethoxy]acetic acid modification, at a loadingof 10 wt % or greater, or 20 wt % or greater, or 30 wt % or greater,calculated based on the weight of the wet nanocrystal cake. While thesuspension is stirred, the capping agent is added to it slowly. Theamount of capping agent used is in the presently exemplified embodiment8-60 wt % to the weight of the wet nanocrystal cake. The suspension isallowed to stir at 20-27° C. for 10-30 minutes and then refluxed at theboiling point of the capping solvent for 30-60 minutes. After refluxing,the clear solution is cooled to 50-60° C. slowly. This suspension isthen filtered to remove dusts and aggregates bigger than 200 nm sizes.

The capped nanocrystals are then precipitated out from the cappingsolvent using heptane (2-4 times the mass of the capped solution). Theprecipitated nanocrystals are collected by centrifugation. Thenanocrystal thus collected is dispersed in tetrahydrofuran (THF) andagain re-precipitated using heptane. This process is repeated twice. Thewet cake of nanocrystals collected in the final step is dried undervacuum for at least 12 hours.

We claim:
 1. A light emitting device comprising, in order: anencapsulation layer or a substrate layer, an array of lenses, an arrayof light emitting pixels at least partially covered by the array oflenses, wherein at least one of the lenses covers at least one of thepixels, and the lenses comprise a material with higher refractive indexthan the encapsulation layer or substrate layer, the refractive index ofthe material being in the range of greater than 1.7 to 1.9 at awavelength of 400 nm, wherein the material comprises a nanocompositecomprising inorganic nanocrystals and a polymeric matrix, and whereinthe inorganic nanocrystals comprise ZrO₂, ZnO, MgO, HfO₂, Nb₂O₅, Ta₂O₅,or Y₂O₃.
 2. The device of claim 1, wherein the at least one lens whichcovers at least one of the pixels comprises a light emitting diode (LED)or an organic light emitting diode (OLED).
 3. The device of claim 1,wherein the device further comprises a planarization layer between thearray of lenses and the encapsulation or substrate layer.
 4. The deviceof claim 1, wherein the device further comprises a transparent conductorlayer between the array of lenses and the array of light emittingpixels.
 5. The device of claim 1, wherein at least one lens of the arrayof lenses at least partially covers more than one pixel of the array oflight emitting pixels.
 6. The device of claim 1, wherein the shape ofthe lenses of the array of lenses are at least one of spherical,semi-spherical, hyper-semispherical, parabolic, concave, convex, orsub-wavelength pyramid array, and/or comprise a surface texture, orother surface curvature, or a Fresnel lens.
 7. The device of claim 1,wherein lenses of the array of lenses comprise a graded or gradientindex profile along at least one dimension of the lens.
 8. The device ofclaim 7, wherein the graded or gradient lens comprises a curved surface9. The device of claim 5, wherein the at least one lens comprises atleast one lens element selected from the group consisting of a singletlens, a lens with graded or gradient index profile, an achromatic lensdoublet, a prism, a filter, a polarizer, a reflector and a Fresnel lens.10. The device of claim 1, wherein each of the lenses is separated froman active region of the light emitting device by less than thewavelength of the highest energy photons emitted by the light emittingdevice.
 11. The device of claim 1, wherein the nanocomposite was UVcurable.
 12. The device of claim 1, wherein the nanocomposite wasthermally curable.
 13. The device of claim 1, wherein the inorganicnanocrystals comprise ZrO₂.
 14. The device of claim 1, wherein theinorganic nanocrystals are smaller than 10 nm in at least one dimension.15. The device of claim 1, wherein the polymer matrix comprises amaterial selected from the group consisting of acrylic, epoxy, silicone,siloxane, polycarbonate, polyurethane, polyimides,poly(3,4-ethylenedioxythiophene) (PEDOT), poly(styrene sulfonate) (PSS),doped PEDOT, polyethylene terephthalate (PET), polyethylene naphthalate(PEN), doped poly(4,4-dioctylcyclopentadithiophene), and theircorresponding monomers and/or oligomers.
 16. An OLED display comprisinga device of claim
 1. 17. A process of making a display comprisingincorporating a device of claim 1 in to a display device.
 18. An arrayof nanocomposite lenses comprising a material with higher refractiveindex than an encapsulation layer or substrate layer of a light emittingdevice comprising an encapsulation layer or a substrate layer and thearray of lenses, wherein the material comprises a nanocompositecomprising inorganic nanocrystals and a polymeric matrix, and whereinthe inorganic nanocrystals comprise ZrO₂, ZnO, MgO, HfO₂, Nb₂O₅, Ta₂O₅,or Y₂O₃.
 19. The array of claim 18, wherein the shape of the lenses ofthe array of lenses are at least one of spherical, semi-spherical,hyper-semispherical, parabolic, concave, convex, or sub-wavelengthpyramid array, and/or comprise a surface texture, or other surfacecurvature, or a Fresnel lens.
 20. The array of claim 18, wherein lensesof the array of lenses comprise a graded or gradient index profile alongat least one dimension of the lens.
 21. The array of claim 20, whereinthe graded or gradient lens comprises a curved surface.
 22. The array ofclaim 18, wherein the lenses of the array of lenses comprise lenselements selected from the group consisting of a singlet lens, a lenswith graded or gradient index profile, an achromatic lens doublet, aprism, a filter, a polarizer, a reflector and a Fresnel lens.
 23. Thearray of claim 18, wherein the nanocomposite is UV cured.
 24. The arrayof claim 18, wherein the nanocomposite is thermally cured.
 25. The arrayof claim 18, wherein the inorganic nanocrystals are smaller than 10 nmin at least one dimension.
 26. The array of claim 18, wherein thepolymer matrix comprises a material selected from the group consistingof acrylic, epoxy, silicone, siloxane, polycarbonate, polyurethane,polyimides, poly(3,4-ethylenedioxythiophene) (PEDOT), poly(styrenesulfonate) (PSS), doped PEDOT, polyethylene terephthalate (PET),polyethylene naphthalate (PEN), dopedpoly(4,4-dioctylcyclopentadithiophene), and their corresponding monomersand/or oligomers.
 27. A telephone, tablet computer, laptop computer,television, watch or display device comprising the device of claim 1.28. A telephone, tablet computer, laptop computer, television, watch ordisplay device comprising an array of claim
 18. 29. A telephone, tabletcomputer, laptop computer, television, watch or display devicecomprising the display device of claim
 17. 30. An organic light emittingdiode (OLED) display comprising: an array of light emitting pixels, andan array of lenses, wherein the array of lenses comprises ananocomposite, wherein the nanocomposite comprises inorganicnanocrystals and has a refractive index between 1.6 and 1.9 at awavelength of 400 nm; and wherein at least one of the lenses is arrangedso as to enhance overall light extraction efficiency from at least onepixel of the array of light emitting pixels, and wherein the array oflenses are not in contact with a planarization layer and not in contactwith a substrate layer.
 31. The OLED display of claim 30, wherein thenanocomposite is UV cured.
 32. The OLED display of claim 30, wherein thenanocomposite is thermally cured.
 33. The OLED display of claim 30,wherein the nanocomposite comprises ZrO₂, ZnO, MgO, HfO₂, Nb₂O₅, Ta₂O₅,or Y₂O₃ nanocrystals.
 34. The OLED display of claim 33, wherein thenanocrystals have a size of less than 10 nm in at least one dimension.35. The OLED display of claim 30, wherein the nanocomposite comprises apolymer, wherein the polymer optionally comprises an acrylic, an epoxyor a silicone.
 36. The OLED display of claim 35, wherein the polymercomprises a siloxane, a polycarbonate, a polyurethane, a polyimide,poly(3,4-ethylenedioxythiophene) (PEDOT), poly(styrene sulfonate) (PSS),polyethylene terephthalate (PET), polyethylene naphthalate (PEN), orpoly(4,4-dioctylcyclopentadithiophene).
 37. The OLED display of claim30, wherein at least one of the lenses covers a single pixel of thearray of light emitting pixels.
 38. The OLED display of claim 30,wherein at least one of the lenses covers multiple pixels of the arrayof light emitting pixels.
 39. The OLED display of claim 33, wherein alens of the array of lenses comprises a spherical surface, asemi-spherical surface, a hyper-hemispherical surface or a parabolicsurface.
 40. The OLED display of claim 30, wherein a lens of the arrayof lenses comprises a concave surface, a convex surface, asub-wavelength pyramid array surface or a textured surface.
 41. The OLEDdisplay of claim 30, wherein a lens of the array of lenses comprises agraded or gradient index profile along at least one dimension of thelens.
 42. The OLED display of claim 30, wherein the array of lenses isseparated from the array of light emitting pixels by a distance of lessthan a wavelength of the highest energy photons emitted by the array ofpixels.